The present invention relates to the treatment of waste liquor by a system which uses the heat that is normally wasted from the cooling of industrial process gas, such as coke oven gas, oil gas, or coal gas, to provide energy for the fractional evaporation of the liquor which condenses from the gas.
In the prior art the treatment of waste liquors that condense from the cooling of industrial process gas has been inadequate in that the product was not environmentally suitable for discharge to the waterways of the nation. Furthermore, the process of treating the liquors consumed valuable chemicals and gave rise to sludges which created a solid-waste disposal problem.
In an effort to overcome the inadequate treatment, to avoid the purchase of valuable chemicals, and to obviate the need for sludge disposal, a prior process was developed to fractionally distill the waste into components of gaseous vapors, purified liquor and concentrated brine. The process achieved its objectives; however, it consumed a large amount of energy and contained a substantial area of heat exchange surfaces that were subject to fouling and corrosion, thereby shutting down the process at frequent intervals. The prior art will be discussed below in specific relationship to coke oven gas, coal gas and oil gas technology. However, it is also applicable to other process gas technology.
Coke is made by the destructive distillation of coal in the absence of air. The heating process, which attains temperatures as high as 2000 F. drives off both the surface water and the water of combination from the coal. Cooling of the gas, which evolves from the heated coal, takes place by means of sprays in the gas collecting mains. The non-condensed gas and vapors leaving the mains, at a temperature of 165 to 180 F., require further cooling to approximately 95 F. Thus further cooling takes place in a gas cooler, known as a primary gas cooler. The purpose of the further cooling is to remove tar vapors and a major portion of the water vapor and to reduce both the volume and temperature of the gas before its admission to the exhauster, which draws the gas from the coke ovens. The vapors that condense are known as waste ammoniacal liquor.
Gas from coal is made by reacting heated coal air and steam. In the air reaction, the amount of air is regulated so that the combustion is essentially incomplete, thereby producing mainly carbon monoxide and nitrogen. In the steam reaction, the principal products are hydrogen and carbon monoxide. The first reaction produces heat which is absorbed in the second reaction.
Fuel gases made from coal vary in calorific value from approximately 150 to 550 BTU per cu. ft. The calorific value may be varied by using oxygen instead of air, or by enriching the gas with vapors from an oil cracking process, or by both in conjunction. The process may incorporate a waste-heat boiler for recovering energy. Oil gasification, such as the carbonization of oil at high temperatures, produces a gas having a calorific value essentially equal to that of natural gas, i.e., approximately 1000 BTU per cu. ft.
The various processes for making gas from coal and oil produce a gas containing components such as water and tar. The total gas requires cooling and cleaning prior to its ultimate use. Cooling takes place in a waste-heat boiler followed by a primary cooler, or solely in a primary cooler. As in the case of coke oven gas, during cooling, tar vapors and a major portion of the water vapor are condensed and the gas is reduced in volume. An exhauster removes the uncondensed gases. Uncondensed vapors remain as waste liquors.
There are two types of primary coolers. One is the direct primary cooler in which heat transfer takes place by direct contact between the gas and the previously condensed, separately cooled waste liquor. In the indirect primary cooler, heat exchange takes place through the medium of tubing.
Waste liquor is usually discharged to a receiving body of water such as a river or a lake. Its usual flow is 15 to 35 gallons per ton of coal which is charged into the coke ovens, 40 to 60 gallons per ton of coal used to produce coal gas, and about one quart per gallon of oil converted to gas. It is highly toxic and must be treated before it is environmentally acceptable for discharge.
The composition of waste liquor from coke oven gas production varies, depending on the nature of the coal from which it is derived, the type of coke oven that is used, and the coking temperature. A typical range of composition expressed in milligrams per liter is listed below:
______________________________________ Phenolics 300 to 4000 Free ammonia (separable by steaming) 1300 to 2000 Fixed ammonia (requires chemicals for its separation) 2600 to 4000 Carbonate 2300 to 2600 Cyanide 10 to 100 Thiocyanate 50 to 500 Total dissolved solids 4000 to 13000 ______________________________________
The composition of waste liquor from coal gas and oil gas production varies according to the gasification process and the type of coal or oil which is employed. A typical composition for waste liquor produced during coal gasification is:
______________________________________ Specific Gravity 1.01 Sulfate 2.2 Grams per liter Chloride 0.9 Grams per liter Sulfide 0.5 Grams per liter Phenol 1.9 Grams per liter Carbon dioxide 3.6 Grams per liter Free Ammonia 0.03 Grams per liter Fixed Ammonia 0.02 Grams per liter ______________________________________
Waste liquor from oil gas production contains about 0.05% hydrocarbons corresponding to the composition of light tars. Other components will be in very small amounts.
The most common method of treating waste liquor from coke oven gas production for discharge is to pass it through a free still, a fixed still, and an activated sludge plant. The free still causes the evolution of free ammonia and certain acidic gases due to direct contact with live steam. The fixed still causes the evolution of fixed ammonia due to the addition of a basic chemical compound such as lime or sodium hydroxide and further direct contact with live steam. The activated sludge plant removes nearly all the phenolics and 25 to 60 percent of the cyanide and thiocyanate.
A well-operated plant, such as the one just described, will produce an effluent containing less than 1 milligram per liter of phenolics and acceptable levels of ammonia. However, the content of cyanide and thiocyanate will be above the toxic limits and the level of total dissolved solids will be essentially undiminished. The color of the liquid will be dark brown and will taste bed. It will also be necessary to provide an environmentally acceptable land area for receiving the lime sludge from the fixed still as well as the biological sludge from the activated sludge plant.
Waste water from coal and oil gasification is commonly passed through a sand filter and an activated carbon filter to remove the organic components. Any inorganic components from coal gasification are not removed from the waste water. For complete treatment, a biological oxidation step can be included.
A newer process has been recently developed to provide a better quality effluent and to avoid, in part, the problem of disposing of the sludge. The central unit in this process is a free ammonia still combined with a multiple effect evaporator. Three effluents are produced. The first is gaseous vapors consisting of ammonia, certain acidic gases, and water vapor. These are incinerated and pass to the atmosphere in an environmentally acceptable manner. The second is purified water which contains most of the phenolics, the reason for the lack of separation being that the boiling points of water and the phenolics are close to one another. The phenolics are further treated by an activated sludge plant, or extracted as a usable by-product. The third is concentrated brine which contains the fixed ammonia, cyanide, thiocyanate, and other dissolved solids. This is incinerated and, in a scrubber unit which is incorporated with the incinerator, recovered as a useful acid. The composition of the acid is principally hydrochloric acid and contains a small percentage of sulphuric acid. Incineration of the raw, dilute waste liquor was also tried, but it required an excessive amount of energy.
The newer process has produced an environmentally acceptable effluent as well as a usable acid. It has two disadvantages. The first is that of fouling and corrosion on the heat exchange surfaces. This occurred despite the fact that the waste liquor was decanted and then filtered before admitting it for treatment and despite the refractory material that was used for the heat exchange tubing, titanium having been the material of choice. Plant shut-downs to correct the results of the fouling and corrosion have been frequent. The second is that of high consumption of energy that is required for distillation, despite the employment of multiple effects to achieve maximum conservation of energy.
A purpose of the invention is to separate the components of gaseous vapors, purified water, and concentrated brine from waste liquors which condense from the cooling of industrial process gas, so that the components may be utilized or treated in an efficient manner.
A further purpose is to separate the components of gaseous vapors, purified water and concentrated brine from waste liquors which condense from the cooling of industrial process gas and to utilize the heat from the cooling of said gas to provide the energy for separating the components from the liquor.
A further purpose of the invention is to separate the components of gaseous vapors, purified water and concentrated brine from the waste ammoniacal liquor which condenses in the primary gas coolers of a coke oven gas, oil gas, or coal gas by-product plant so that the components may be used or treated in an efficient manner and to do so utilizing the thermal energy which is normally wasted from the primary gas coolers.
A further purpose of the invention is to provide a system for separating the components of gaseous vapors, purified water and concentrated brine from waste liquors in which the principal mechanism of heat transfer to or from the foul gas or foul liquor is by direct contact, thereby avoiding corrosion and fouling of heat exchange surfaces.
A further purpose of the invention is to provide a system for separating the components of gaseous vapors, purified water and brine from waste liquors in which the salt in the brine is crystallized and withdrawn from the brine in the crystal form.